1. Field of the Invention
The present invention relates generally to a mechanism to assist the operation of a tarp system used to cover an open top trailer, container or similar structure. More particularly, the invention relates to mechanisms for facilitating the rolling or unrolling of a tarp between a stored, rolled up condition and the unfurled condition covering the open top.
2. Description of the Prior Art
Various types of truck, trailer and container bodies are designed to be loaded through an open top. Such bodies are typically used to haul grain, earth or refuse. It is desirable to use tarpaulins or other coverings to close the open tops of such containers. The tarpaulins serve to shelter the truck, trailer or container contents against the elements and to maintain the contents in the body when it is exposed to high winds or moved from place to place.
Several systems are known for covering truck, trailer and containers with tarpaulins. Typically, the covering is unrolled from a long tube by associated metal hardware from an open position in which the covering is entirely wrapped around the tube, exposing the interior of the container, to a closed position in which the covering is spread over the top of the container. These systems tend to use metal bows to span the width of the opening and support the covering at regular parallel spaced intervals along the length of the open top.
An example of such a system is shown in U.S. Pat. No. Re. 31,746 issued to Dimmer et al. As shown in the patent, a covering is attached along one lengthwise edge to a tube which, in turn, is connected by a universal joint to a crank. The universal joint can be removably attached to the roll tube by a spline and pin or other similar mechanism. An operator at ground level can turn the crank causing the covering to roll up lengthwise on the tube and uncover or cover the top of the trailer or container. The end of the roll tube to which the universal joint and crank arm are attached can be described as the active end because the torque or turning force is applied there. Conversely, the opposite end can be described as the passive end because the torque is applied indirectly through the length of the tube.
A feature of the system shown in U.S. Pat. No. Re. 31,746 (and the commercial embodiment thereof) is an elastic cord (item 78) attached at one end to a narrower plastic (PVC) tube within the roll tube and at the other end to a forward edge of the latch plate (or, alternatively, to the inside or outside of the body). (See U.S. Pat. No. Re. 31,746, column 4, lines 49-66.) The elastic cord pulls the passive end of the tube toward the latching side of the container to tension the end of the roll bar opposite the direct rolling force provided on the active end by the crank. The tensioning force decreases as the roll bar is moved closer to the latching side, progressively unrolling more of the covering material over the opening.
In another variation of such a system, as disclosed in U.S. Pat. No. 5,487,584 issued to Jespersen shows a series of bows that are at least partially displaceable from over the opening. In that system, the tarpaulin rolls up lengthwise along a side of the container and the bows to expose the opening. The bows can be pivoted at least partially out of the way of the container opening to increase accessibility of the interior of the body. The tarp is closed over the opening, after the bows are swung back to a position spanning the opening by unrolling the covering material from the roll bar across the top in a conventional manner.
In some circumstances, the use of the resilient cord at the passive end of the roll bar causes the passive side to be advanced at a different rate across the opening than the rate of the active end of the roll bar. In essence, the pulling by the elastic cord applies a different force applied in a different direction than the turning/rolling force applied directly at the opposite end by the crank arm. During the opening operation, the imbalance of forces can cause the covering material to be wrapped more tightly around the passive side of the bar or tube than the material on the active side. This will result in the passive side progressing more slowly than the active side. Conversely, the passive side tends to advance faster than the active side of the roll or tube during the closing operation, occasionally resulting in stationary unwinding of the covering on the bar at the active end. Such uneven rolling may cause the operator to try to use the crank arm to slide, push or pull the roll bar and associated covering material to advance the active end of the roll bar, or to move it over a heaped load or other blockage, for example.
It is an object of the invention to provide an assist mechanism for a tarp system whereby a more equal balance of tension forces applied to the roll bar is achieved. It is another object of the invention to provide an apparatus where uneven unrolling is less likely to occur or is avoided altogether. It is a further object of the invention to provide supplemental tension for a tarp system to increase total tension forces at the roll bar of a roll-up tarp system. Still further it is desirable to provide an assist mechanism which may be retrofitted readily to existing covering apparatus to improve the rolling operation.